The present invention relates to a method of manufacturing a connecting rod for an internal combustion engine and more particularly to a method of manufacturing a connecting rod having a bundle of uni-directional inorganic fibers filled in and combined with a matrix of light alloy in the longitudinal center portion of the rod.
The fiber-reinforced connecting rod shown in FIGS. 3 and 4 has been previously proposed and comprises a small annular end portion, a large annular end portion B and a rod portion R connecting the small and large annular end portions.
The rod portion R has a core made of a bundle of uni-directional inorganic fibers filled in and combined with a light alloy matrix, and a single component matrix portion formed outside of the core. The small end portion can be considered to have a central axis. In a cross-sectional view of the rod portion R in FIG. 4, the single component matrix portion includes a pair of rectangular portions M.sub.1 formed along the central axis of the small end portion in such a manner as to sandwich the core of fibers and alloy and a pair of arc-shaped portions M.sub.2 formed between the rectangular portions M.sub.l and also sandwiching the core of fibers and alloy. Notches are formed between the rectangular portions and the arc-shaped portions. The concentration of stress is higher at the notches and, therefore, a factor of stress concentration is relatively increased. The single component matrix portion is made by squeeze casting and, therefore, the strength of the rod portion is higher than other general cast articles. However, it has been found in the practice of a fatigue test that the single component matrix portion was inclined to have an initiation of a fatigue crack. The crack extended all over the single component matrix portion to surround the core of the bundle of fibers. Finally, it caused a premature fatigue failure. It has been found further that when such a single component matrix portion had a defect in casting, for example, a mixing of oxidated substance, a blow-hole, etc., the fatigue strength was liable to be variable. As a result, the lifetime of the single component matrix portion could not be kept constant or predicted.